1) Field of the Invention
The present invention relates to a developing device that develops a latent image held on a latent-image carrying unit by a two-component developer which includes toner and a magnetic carrier, an image forming apparatus such as a copying machine, a facsimile, a printer which uses the developing device, and a process cartridge.
2) Description of the Related Art
So far, image forming apparatuses with two-component developing technology in which a two-component developer that contains toner and a magnetic carrier have been known. In this type of image forming apparatus, a developing device that develops a latent image which is held on a latent image carrying unit such as a photosensitive drum, moves a developer carrying unit such as a developing roller etc. in a predetermined direction. With this movement, the developer carrying unit carries a two-component developer in a developer receptacle to a position opposite to the latent image carrying unit. After the two-component developer is used for developing of a latent image, it is returned to the developer receptacle. After the two-component developer that is returned to the developer receptacle is replenished with suitable amount of toner by a toner replenishing unit, it is again held on the developer carrying unit and used for developing. At this time, the newly replenished toner is stirred with the magnetic carrier in the developer receptacle. Due to this stirring, the magnetic carrier particles are rubbed against each other on the developer carrying unit, thereby causing frictional charging.
While there is an advancement in reduction in size of toner with the improvement in image quality in recent years, in such image forming apparatuses which use the two-component developing technology, there is a tendency of forming an image in which there is the so called excessive concentration of toner in which non-image portion is contaminated due to the scatter of toner from a surface of the magnetic carrier. In general, as the toner particle size becomes smaller, the charging is affected. Therefore, the toner is not charged sufficiently after the developer receptacle is replenished with the toner till it is carried to the position opposite to the latent image carrying unit and the toner tends to be scattered due to insufficient charging.
To solve such a problem, in Japanese Patent Application Laid-open Publication No. H11-237761, a toner in which the charging is improved by regulating a peak value and a bottom value of a frequency distribution of a q/d value (where q is an amount of electric charge and d is a diameter of a toner particle) in a predetermined range, has been proposed.
Moreover, in an image forming apparatus equipped with a developing device, during developing, if the toner is adhered on a surface of the latent image carrying unit other than a latent image portion in a developing region, there is a so called contamination of the surface of the medium. The main cause for the contamination is supposed to be faulty charging of toner. In other words, if insufficiently charged toner is carried to the developing region, the toner is adhered on a non latent-image portion where it is not supposed to be adhered, and this toner becomes a contamination on the surface of a medium. Developing units disclosed in Japanese Patent Application Laid-open Publication No. H9-325612 and Japanese Patent No. 2834747 in which such a type of contamination can be suppressed, have been known. In any of these developing devices, toner with which the developing device is replenished, is charged in advance and then the developing device is replenished with this charged toner. This enables to suppress contamination caused due to use of toner that is used for replenishment without being charged sufficiently.
However, in toner disclosed in Japanese Patent Application Laid-open Publication No. H11-237761, during the period of time after the developer receptacle is replenished with toner till it is carried to the position opposite to the latent image carrying unit, the charging is not sufficient necessarily. This is because for charging of the toner, not only the charging capability of the toner but also the parameters related to the charging of the toner such as stirring of the two-component developer, friction characteristics of the magnetic carrier with respect to the toner particles contribute to a great extent. Therefore, even if it is toner disclosed in the Japanese Patent Application Laid-open Publication No. H11-237761, according to the setting of the stirring, there is a possibility that the toner is carried to the position opposite to the latent image carrying unit before being charged sufficiently and scattered conspicuously.
Therefore, the applicant(s) of this patent application in the Japanese Patent Application No. 2003-030128 has (have) proposed an image forming apparatus in which parameters related to the charging of toner in the developing device are set such that the number proportion of toner particles that fulfill the following relational equation A1 in a two-component developer which is held on a surface of a developer carrying unit, is in a range of 0 to 10%.
                              2500          ⁢          m                >                  μ          ⁢                                          ⁢                      0.75                          4              ⁢                                                          ⁢              π              ⁢                                                          ⁢                              ɛ                0                                              ⁢                                    (                              q                r                            )                        2                                              (        A1        )            where m: mass of toner particleμ: coefficient of friction between toner and magnetic carrierπ: ratio of circumference of a circle to it's diameterε0: dielectric constant in vacuum (8.85×10−12 [F/m])r: radius of toner particle [m]q: amount of electric charge on toner particle [c]
According to this invention, due to the reason mentioned below, it is possible to control an image in which there is an excessive concentration of toner due to the scatter of toner. On the developer carrying unit, a two-component developer causes a magnetic carrier to erect or to flatten a magnetic carrier that is erected (hereinafter, “flattening”) with a change in the magnetic force that sets the two-component developer towards a surface of the developer carrying unit. When the magnetic carrier is erected or flattened, there is a collision between the particles of the magnetic carrier and shocks of greater or lesser extent are sent to toner that is adhered on surface of the particles of the magnetic carrier. If the inertial force that is developed in the toner due to the shocks becomes greater than adhesion between the toner and the magnetic carrier, the toner is separated from the surface of the magnetic carrier and scattered. Thus, since for the insufficiently charged toner particles, the inertial force becomes greater than the static electrical adhesion, the relational equation A1 is derived. On the developer carrying unit, if the proportion of the toner particles which fulfill the relational equation A1 becomes greater, the scatter of the toner increases to an extent such that the excessive concentration of toner in the image becomes conspicuous. Based on such a consideration, experiments were carried out and a relationship between a proportion of toner particles which fulfill the relation mentioned above and scatter of the toner was examined. As a result of the experiment it was found that if the number percentage of the insufficiently charged toner particles which fulfill the relationship mentioned above, is controlled below 10 [%], it is possible to reduce the scatter of toner to an extent that there is no excessive concentration of toner.
Thus, in the patent application mentioned above, the scatter of the toner is reduced by reducing the proportion of toner with less charging in the two-component developer that is held on the surface of the developer carrying unit by regulating the amount of electric charge q in toner particle and the diameter d of toner particle. However, the level of scatter of the toner in the actual developing device, is affected not only by the proportion of the insufficiently charged toner in the two-component developer that is held on the surface of the developer carrying unit but also by the linear velocity of the developer carrying unit, the amount of developer that is held on the developer carrying unit, the size of opening of the developing device etc. Concretely, the scatter of the toner increases with increase in the linear velocity of the developer carrying unit, increase in the amount of developer held by the developer carrying unit, rise in the density of the toner, and widening of the area of the opening, and increase in the density of the toner. Therefore, taking into consideration the effect of these factors, it would be useful to regulate the conditions that control the scatter of toner in the developing device.
Even if it is a toner according to Japanese Patent Application Laid-open Publication No. H11-237761, in reality, proportion of inclusion of the toner with less charging and toner with big particle size is high. As it is mentioned in the latter part, the toner with less charging and the toner with big particle size has weak electrostatic adhesion with a carrier. Therefore, even if it is a toner according to Japanese Patent Application Laid-open Publication No. H11-237761, there is a possibility of conspicuous scatter of the toner. Moreover, in the Japanese Patent Application Laid-open Publication No. H11-237761, effect of a change in the movement of a magnetic brush that is formed on the developer carrying unit on the toner is not taken into consideration.
According to the conventional technology, even if the toner is charged sufficiently, there is a possibility of contamination of the surface of the medium. This is due to the reason given below.
In a case of using the two-component developer, due to the electrostatic attraction between toner and a magnetic carrier (toner carrier) that is charged with a charge of polarity opposite to that of the toner, the toner is carried to a developing region in a state of being adhered on the magnetic carrier. When the toner is carried, if the amount of toner adhered on the magnetic carrier is large, in a part of the toner, there are particles which have weak electrostatic attraction with the magnetic carrier. If toner having weak electrostatic attraction with the magnetic carrier is carried to the developing region, even if the toner is charged sufficiently, in the developing region, it falls apart from the magnetic carrier and is adhered on a non latent-image portion of the latent image carrying unit. The contamination is considered to be occurring as a result of this.
The phenomenon mentioned above is not limited to the two-component developer and can occur similarly even if it is a one-component developer that includes a toner which does not include a magnetic carrier. In other words, in a case of the one-component developer, the toner is carried to a developing region in a state of being adhered on a surface due to the electrostatic attraction between the toner and a surface of a developing roller that is charged with a charge of polarity opposite to that of the toner. In this case also, when the toner is carried, if the amount of toner adhered on the surface of the developing roller per unit area is large, in a part of the toner, there are particles which have weak electrostatic attraction with the surface of the developing roller. Therefore, even if the toner is charged sufficiently, in the developing region, the toner falls apart from the surface of the developing roller and is adhered on a non latent image portion of the latent image carrying unit, which may cause contamination on a surface of the medium.